Changes in Regioselectivity of H Atom Abstraction during the Hydroxylation and Cyclization Reactions Catalyzed by Hyoscyamine 6β-Hydroxylase

Article abstract of DOI:10.1021/jacs.8b11585

Hyoscyamine 6β-hydroxylase (H6H) is an αKG-dependent nonheme iron oxidase that catalyzes the oxidation of hyoscyamine to scopolamine via two separate reactions: hydroxylation followed by oxidative cyclization. Both of these reactions are expected to involve H atom abstraction from each of two adjacent carbon centers (C6 vs C7) in the substrate. During hydroxylation, there is a roughly 85:1 preference for H atom abstraction from C6 versus C7; however, this inverts to a 1:16 preference during cyclization. Furthermore, ¹⁸O incorporation experiments in the presence of deuterated substrate are consistent with the catalytic iron(IV)-oxo complex being able to support the coordination of an additional ligand during hydroxylation. These observations suggest that subtle differences in the substrate binding configuration can have significant consequences for the catalytic cycle of H6H.

Full text of DOI:10.1021/jacs.8b11585

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Article
Changes in regioselectivity of H atom abstraction during the hydroxylation
and cyclization reactions catalyzed by hyoscyamine 6#-hydroxylase
Richiro Ushimaru, Mark W. Ruszczycky, and Hung-wen Liu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11585 • Publication Date (Web): 14 Dec 2018
Downloaded from http://pubs.acs.org on December 14, 2018
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Changes in regioselectivity of H atom
abstraction during the hydroxylation and
cyclization reactions catalyzed by hyoscyamine
6β-hydroxylase
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Richiro Ushimaru,† Mark W. Ruszczycky,∗,‡ and Hung-wen Liu∗,†,‡
†Department of Chemistry, University of Texas at Austin
‡Division of Chemical Biology and Medicinal Chemistry, College of Pharmacy, University of
Texas at Austin
E-mail: mwr8@utexas.edu; h.w.liu@mail.utexas.edu
Abstract
Hyoscyamine 6β-hydroxylase (H6H) is an αKG-dependent nonheme iron oxidase
that catalyzes the oxidation of hyoscyamine to scopolamine via two separate reac-
tions: hydroxylation followed by oxidative cyclization. Both of these reactions are
expected to involve H atom abstraction from each of two adjacent carbon centers (C6
versus C7) in the substrate. During hydroxylation there is a roughly 85 : 1 preference
for H atom abstraction from C6 versus C7; however, this inverts to a 1 : 16 preference
during cyclization. Furthermore, 18O incorporation experiments in the presence of
deuterated substrate are consistent with the catalytic iron(IV)-oxo complex being able
to support the coordination of an additional ligand during hydroxylation. These ob-
servations suggest that subtle differences in the substrate binding configuration can
have significant consequences for the catalytic cycle of H6H.
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O2 + αKG
H6H (FeII)
(1)
(1)
7
7
7
7
6
6
6
H6H
H6H
1
H6H
H6H
5
4
2
product
dissociation
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(2)
7
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O2 + αKG
H6H (FeII)
7
6
(3)
product
dissociation
(2)
7
6
7
7
H6H
H6H
6
6
8
9
3
Figure 1: Mechanistic outline of the hydroxylation and cyclization reactions catalyzed by
H6H.
Introduction
Nonheme iron oxidases represent an important class of enzymes that catalyze a wide
range of oxidative transformations.1–4 The prototypical catalytic cycle of these enzymes
involves the use of molecular oxygen to oxidize α-ketoglutarate (αKG) coordinated to
an active site iron(II) center (4) to generate succinate, CO2 and an activated iron(IV)-oxo
complex (see 5 & 8 in Fig. 1).1,5 The iron(IV)-oxo complex then participates in the oxida-
tion of the nominal substrate with the possible involvement of organic radicals confined
to the active site. Despite the reactivity of iron(IV)-oxo complexes and radical interme-
diates, nonheme iron oxidases are able to exert considerable control over the course of
the reaction such that they proceed in a regio- and stereoselective manner. Therefore,
the mechanistic study of these enzymes can offer significant insights into how enzymes
constrain the reactions they catalyze.
Hyoscyamine 6β-hydroxylase (H6H) is one such enzyme of particular interest. H6H
is an αKG-dependent nonheme iron oxidase from plants that catalyzes two separate and
sequential oxidations during biosynthesis of the antimuscarinic natural product scopo-
lamine (3).6–13 The first of these reactions is the C6-hydroxylation of hyoscyamine (1) to
generate 6β-hydroxyhyoscyamine (1→2, see Fig. 1). This is followed by a second reaction
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where H6H catalyzes the dehydrogenation of 2 to yield the epoxide scopolamine (2→3).
These two reactions are believed to involve H atom abstraction from the exo-C6 and exo-
C7 positions of the corresponding substrates 1 and 2, respectively, by an iron(IV)-oxo in-
termediary species (5 & 8) as shown in Fig. 1. However, H6H can also accept the regioiso-
mer 10 (see Fig. 2) as a substrate for the cyclization reaction again yielding scopolamine.14
These observations immediately raise questions regarding how the regiochemistry of the
two reactions is controlled within the H6H active site.
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Figure 2: Additional H6H substrate and product analogs studied in the present work.
Analysis of the H6H-catalyzed hydroxylation reaction by LC-MS demonstrates that
over 98% of the product is 2 consistent with previous reports.7 However, as described in
the Supporting Information, a minor side product can also be identified and was confirmed
to be 10. Therefore, while H atom abstraction from C7 is possible during hydroxyla-
tion, nearly all of the reaction flux proceeds via H atom abstraction from C6. It follows
that from a biological standpoint 2 rather than 10 is the relevant biosynthetic substrate
for cyclization to 3 and thus exo-C7–H rather than exo-C6–H is almost always the only
option for H atom abstraction during cyclization. However, the regioselectivity of H6H-
catalyzed hydroxylation suggests that the H6H iron(IV)-oxo intermediate bound with
2 (i.e., 8) would actually undergo H atom abstraction and subsequent cyclization more
slowly than the corresponding complex with 10. In other words, the preferred site of H
atom abstraction may in fact be C6 rather than C7 during cyclization as well. In contrast,
preferential H atom abstraction from C7 during cyclization would indicate an inversion in
the regioselectivity of H6H-catalyzed radical initiation via an unknown mechanism—the
details of which underlie precisely how the iron(IV)-complex interacts with the substrate
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thereby directing the course of the reaction.
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Results and Discussion
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In order to properly address the question of regioselectivity during the H6H-catalyzed
reactions, it is necessary to disentangle the kinetics of H atom abstraction from all other
elementary reactions (e.g., substrate binding) in the two catalytic cycles of H6H. In the
case of hydroxylation, H atom abstraction is product-determining, and thus at pseudo-
steady-state the 10/2 product ratio is a constant under fixed concentrations of αKG and
O2 and given by
07
k
k
[10]
[2]
hyd
7/6 0
k
=
=
.
(1)
hyd
06
hyd
07
06
In this expression, k
and k
represent the net rate constants15 associated with the net
hyd
hyd
decomposition of the iron(IV)-oxo complex bound to 1 via hydroxylation at C7 versus C6,
respectively (i.e., 5→10 and 5→2, see Fig. 3). In other words, the net rate constants are
indicative of all steps from H atom abstraction up to and including the first “irreversible”
7/6 0
downstream elementary reaction. Therefore,
k
reports on the relative partitioning
hyd
between the two hydroxylation pathways (see Fig. 3).
The partitioning ratio 7/6k0hyd was measured by stirring 16 µM H6H in a well-ventilated
vial under air (0.21 atm O2) with 100 µM 1 in the presence of 10 mM αKG, 400 µM FeSO4,
4 mM sodium ascorbate and 50 mM sodium 4-(2-hydroxyethyl)-1-piperazineethanesul-
fonate (HEPES buffer) at pH 6.9 and room temperature. Under these conditions, the con-
centrations of the cosubstrates αKG and O2 remain essentially unchanged as the substrate
1 is consumed. Aliquots were taken at three time points within 20 min of initiating the
reaction and subjected to LC-MS analysis whereupon the product peaks in the extracted
ion chromatograms (EICs) were integrated to provide the respective signal intensities.
Samples were diluted to ensure that both the 2 (high intensity) and 10 (low intensity) sig-
nals were simultaneously within the linear dynamic range of the instrument; however,
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(1)
k7hyd
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k6hyd
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7
6
7
14
6
6
kh′y6d
kh′y7d
7
6
7
(2)
(15)
6
15
2
10
Figure 3: Partitioning model for the H6H-catalyzed hydroxylation of hyoscyamine (1)
across the product determining step of H atom abstraction. The elementary reactions
involving H atom abstraction from the exo-C6 versus exo-C7 sites are governed in the
“forward” direction by the pseudo-first order rate constants k6hyd versus k7hyd, respectively.
The steady-state decomposition of intermediate 5 to generate 2 versus 10 is governed by
06
the net rate costants k
and k0h7yd. As discussed in the text, experimental results suggest
hyd
that the 5→6 and 5→14 elementary reactions may be treated as irreversible.
an intercept correction was necessary to account for minimum detection cutoffs. This
correction resulted in an approximately 15% increase in the observed ratios versus the
uncorrected values. Both 1 and 2 were determined to have the same ionization efficiency,
and scopolamine production was found to be negligible. The experiment was replicated
five times yielding an average product ratio of 0.0117 ± 0.0003 (± standard error of the
mean, n = 5). Experimental details including examination of controls are provided in the
Supporting Information.
A similar experiment was also performed with the regio- and stereo-specifically deuter-
ated substrate 11, which was synthesized as described in the Supporting Information. In
this case, the observed 13/2 ratio was found to be 0.50 ± 0.04 (n = 5) under the aforemen-
tioned reaction conditions. By analogy to Eqn. (1), this product ratio is indicative of the
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A
B
7
7
7
–CO2
7
6
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8a
8b
6
(2)
(2)
(2)
(3)
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7
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7
–CO2
(3)
6
6
6
(10)
18b
18a
(10)
19
(10)
Figure 4: Mechanistic hypothesis for the reduction and inversion of the regioselectivity of
H6H-catalyzed cyclization with respect to hydroxylation. This mechanism involves direct
coordination of the substrate to the iron(IV)-oxo complex to facilitate dehydrogenation of
the hydroxylated substrates 2 (A) and 10 (B).
net rate constant ratio given by16
07
06
07
k
k
k
k
k
k
hyd
hyd
hyd
7/6D 0
D 06
7/6 0
k
=
=
·
= khyd
·
k
.
hyd
(2)
hyd
06D
06D
06
hyd
hyd
hyd
In Eqn. (2), k0h6yDd is the net rate constant for steady-state decomposition of the iron(IV)-oxo
D 06
complex via D atom abstraction from C6 of 11, and khyd is the corresponding kinetic
isotope effect. Consequently, this KIE can be readily computed from the ratio
D 06
7/6D 0
7/6 0
k
=
k
/
k ,
hyd
(3)
hyd
hyd
yielding an observed value of 43 ± 3.
While the large primary deuterium KIE on k
exceeds the semiclassical limit,17 it
06
hyd
is consistent with previous reports of H atom abstraction catalyzed by other nonheme
iron enzymes.18–21 Moreover, it supports the hypothesis that the elementary reaction in-
volving H atom abstraction (5→6) is most rate-limiting (i.e., has the greatest sensitivity
index) with respect to decomposition of the iron(IV)-oxo species bound with substrate
to hydroxylated product and may thus be reasonably modeled as “irreversible” in the
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steady-state (i.e., it has a negligible reverse commitment along this sequence).22–26 As-
suming the same holds for H atom abstraction from C7 and decomposition proceeds first
via H atom abstraction (see Fig. 3), we can make the approximations
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D 06
D 6
k
≈ khyd
,
(4a)
(4b)
hyd
06
k0h7yd/k
≈ k7hyd/k6hyd
,
hyd
where k6hyd and k7hyd are the respective microscopic rate constants governing H atom ab-
straction from C6 (5→6) and C7 (5→14).26 These conclusions are consistent with com-
putational results on the mechanisms of non-heme iron oxidases in general.27 Therefore,
7/6 0
k
not only reports on the relative partitioning between the two hydroxylation path-
hyd
ways but the regioselectivity of H atom abstraction itself.
In contrast to hydroxylation, the regioselectivity of H atom abstraction during cycliza-
tion cannot be inferred from the partitioning across a product-determining step. How-
ever, if H atom abstraction is effectively irreversible (see above) and the only elementary
reactions to be significantly perturbed by deuteration or OH placement in the substrate
are substrate binding and H atom abstraction, then the ratio of V-max for 1 versus that
for H6H substrate Y = 1, 12, 2 or 10 (see Fig. 2) can be approximated as
V1/VY ≈ α1k1/kY + (1 − α1).
(5)
Here, k1/kY is the ratio of the microscopic rate constants governing H atom abstraction
from 1 versus Y. The weighting term α1 lies between 0 and 1 and depends only on the
values of the rate constants in the catalytic cycle for 1. The result (5) is trivial when Y = 1
and follows immediately from an analysis by Northrop when Y = 12,22,28 which is gen-
eralized to the case where Y = 2, 10 in the Supporting Information. Note that contributions
to V1/VY from substrate binding are eliminated, because the substrate concentrations are
saturating. Therefore, once the weighting term α1 is known, Eqn. (5) provides a means
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to calculating the regioselectivity of H atom abstraction during cyclization from measure-
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ments of V-max.
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Table 1: Measurements of V-max per unit concentration total enzyme (e0) for 1, 12, 2 and
10 in the presence of 10 mM αKG and 0.21 atm O2 at room temperature and pH 6.9. Values
are reported ± one standard error of the estimate.
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substrate V/e0 (min−1) n
1
12
2
3.90 ± 0.04
0.10 ± 0.01
0.32 ± 0.04
0.020 ± 0.003
3
3
3
3
10
Compounds 2 and 12 were prepared as described in the Supporting Information, and
initial rates were measured for saturating 1, 12, 2 and 10 at fixed 10 mM αKG and 0.21 atm
O2. In these experiments, reactions were initiated by the addition of 1.6 µM H6H to solu-
tions containing 100 µM substrate, 10 mM αKG, 400 µM FeSO4, 4 mM sodium ascorbate
and 50 mM HEPES at pH 6.9 and room temperature in a well-ventilated vial under air.
Reaction aliquots were taken at three time points (< 20% conversion), quenched by addi-
tion to MeCN and analyzed by LC-MS to obtain the signal intensities of the product and
residual substrate. Since the ionization efficiency of 1 is equal to that of 2 (see Supporting
Information), the fraction of reaction in the aliquot could be calculated directly from peak
integrations of the corresponding EICs. In contrast, the ionization efficiency of scopo-
lamine (3) was found to be ca. 50% that of either hydroxylated substrate and was thus
corrected accordingly prior to determining the fraction of reaction. From the fraction of
reaction, the residual substrate concentration could be calculated and linearly regressed
versus quench time to obtain the initial rate. The experiment was replicated three times
for each substrate. Finally, decreasing or increasing the initial substrate concentration by
50% in all cases induced no significant change to the measured initial rates consistent with
substrate saturation and no substrate inhibition at 100 µM for all four substrates under
the experimental conditions.
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The V-max parameters measured for each of the four substrates are listed in Tbl. 1.
We can solve for the weighting term α1 from Eqn. (5) by using the previous result that
7
8
9
D 6
D 06
k
≈ khyd to obtain
hyd
V1/V12 − 1
Dk6hyd − 1
α1 =
= 0.9 ± 0.1.
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As this does not differ significantly from unity, it implies that the primary deuterium
isotope effect on H atom abstraction is almost fully expressed in the V-max parameter.
This conclusion is consistent with the observation based on the values in Tbl. 1 that
D 6
V1/V12 ≈ 40 ≈ khyd
.
Furthermore, since α1 is measurably greater than 0, Eqn. (5) can also be used to get the
ratio
k7cyc
V1/V10 + (α1 − 1)
7/6
k
=
=
.
(6)
cyc
k6cyc
V1/V2 + (α1 − 1)
7/6
Substituting in the observed parameters, we thus obtain a value of 16 ± 3 for
k
.
cyc
Therefore, the preference for H atom abstraction from C7 versus C6 increases roughly
1400-fold in the H6H-catalyzed cyclization reaction compared to hydroxylation. How-
ever, whereas hydroxylation proceeds with a roughly 85 : 1 selectivity for H atom ab-
straction from C6 vs. C7, the magnitude of the selectivity for C7 vs. C6 during cylization
is reduced to ca. 16 : 1. Thus, while there is indeed an inversion in the site-selectivity
between hydroxylation and cylization, the effect is strongest during hydroxylation when
no substrate hydroxyl group is present.
One possible hypothesis to help explain this result is that the hydroxyl groups of 2
and 10 coordinate the ferryl iron during the cyclization reaction as shown in Fig. 4. It has
been established that the activated iron(IV)-oxo complex of a number of nonheme iron
oxidases is pentacoordinate though computations suggest that octahedral complexes are
also feasible.29–32 Consequently, the coordination environment of the iron(IV)-oxo com-
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plexes during H6H-catalyzed hydroxylation versus cylization need not necessarily be the
same. Thus, the complexes poised for H atom abstraction during cyclization (i.e., 8 & 18)
may resemble the putative product complexes expected to follow hydroxyl rebound dur-
ing hydroxylation (i.e., 7 & 15). Coordination of the substrate hydroxyl group would also
permit inner-sphere electron transfer from the substrate to the iron center thereby facilitat-
ing the oxidative cyclizations of 2 and 10. However, given that the ability of iron(IV)-oxo
complexes to catalyze H atom abstraction appears to be highly sensitive to orientation,33
direct coordination of a substrate to the ferryl iron may also serve to reorient the reactive
oxo-ligand to favor H atom abstraction from the adjacent carbon (8b→9 & 18b→19 in
Fig. 4). In comparison with the unligated case during hydroxylation, the result may thus
be a change in preference for abstraction of the C7-H atom but not without the observed
reduction in overall regioselectivity.
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As an initial test of this hypothesis, the ability of the iron(IV)-oxo complex to support
an additional aquo or hydroxide ligand during the H6H-catalyzed hydroxylation reaction
was investigated. When 1 was incubated with H6H and 18O2 in natural abundance wa-
ter, the majority (87%) of the hydroxylated product was found to contain 18O consistent
with a previous report.9 Likewise, in the converse experiment with natural abundance O2
and 18O-water, only 10% of the product contained 18O. However, when the same exper-
iments were conducted with the deuterated substrate 12, 57% of the product contained
18O with 18O2 and natural abundance water becoming 40% in the presence of natural
abundance O2 and 18O-water (see Supporting Information). Hence, there appears to be sig-
nificant exchange of the reactive oxo-ligand in the ferryl complex with solvent during the
H6H-catalyzed hydroxylation reaction. This implies that the iron(IV)-oxo complex can
indeed support an additional ligand,34,35 and is thus consistent with the hypothesis that
the substrate directly coordinates the iron(IV)-oxo complex during the H6H-catalyzed
cyclization reaction while not doing so during hydroxylation.
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Conclusions
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H6H catalysis represents a useful system for studying how the course of enzymatic reac-
tions involving highly reactive intermediates can be controlled. During the H6H-catalyzed
hydroxylation of hyoscyamine, nearly all of the reaction flux proceeds via H atom abstrac-
tion from the exo-C6 position. However, there is more than a thousand-fold inversion in
this regioselectivity when a hydroxyl group is present at the adjacent carbon. One possi-
ble mechanism to explain this inversion involves direct coordination of the hydroxylated
substrate to the Fe(IV)-oxo complex during cyclization (as depicted in 8b and 18b). Thus,
in the absence of coordination, the oxo ligand may be directed so as to favor H atom ab-
straction and subsequent hydroxylation at C6. In the presence of coordination, however,
the reactive oxo ligand may be shifted to abstract the H atom from the adjacent carbon
being most optimized for abstraction of the exo-C7 H atom in particular. Nevertheless,
other explanations for the inversion in regiochemistry that do not necessarily involve di-
rect coordination of the substrate to the iron-center (e.g., substrate repositioning due to
changes in H-bonding, steric interactions or solvent reorganization within the active site)
remain very much open possibilities. Further evaluation of these and other mechanistic
hypotheses regarding the two H6H-catalyzed reactions is thus expected to provide ad-
ditional insights into how nonheme iron enzymes direct the course of the reactions they
catalyze.
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Acknowledgement
We thank Dr. Takashi Hashimoto at Nara Institute of Science and Technology for gener-
ously providing the clone of h6h. This work was supported by grants from the National
Institutes of Health (GM113106 & GM040541) and the Welch Foundation (F-1511).
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Supporting Information Available The Supporting Information is available free of charge
on the ACS Publications website DOI: xx.xxxx/jacs.xxxxxxx. Details regarding H6H as-
says, analytical controls, synthesis of compounds and kinetic analysis.
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Graphical TOC Entry
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H6H
H6H
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